Method of depositing and impervious metal film on a granular surface



May 20, 1952 v. c. CAMPBELL 2,597,617

METHOD OF' DEPOSITING AN IMPERVI METAL -FILM O GRANULAR SURF' Fil NOV. 9, 1946 Il@ rwtorf: 'Vorw Campbell,

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Patented May 20, 1952 METHOD OF DEPOSITING AN IMPERVIOUS .METAL FILM ON A GRANULAR SURFAQE `Von C. Campbell, Scotia, N. Y., assigner to `General Electric Company, a corporation-of New lork Application November 9, 1946, Serial No. l709,037

2 anims. l

My invention `relates to an improved method of depositing a smooth film on a granular surface and specically to an improved method of applying an impervious metal film on the iiuorescent screen of a cathode ray tube.

In the operation of cathode ray tubes in tele- `vision receivers, considerable difficulty is encountered as a -result of darkening or burning of the fluorescent screen by ions which are present in the electron beam. It is known that this difculty may be avoided by applying a thin, impervious metal backing over the uorescent screen. Known methods for applying coatings of this type have not been well adapted for factory production and have failed to produce a thin coating which is sufficiently free from perforations. The present invention provides a method which overcomes both of these defects.

It is an object of my invention to provide. an improved method of depositing a smooth iilm on a granular surface.

It is another object of my invention to provide an improved method of applying a thin conducting coating on the fluorescent screen of a cathode ray tube.

Further objects and advantages of my invention will become apparent as the following description proceeds, reference being had to the accompanying drawing, and its scope will be pointed out in the appended claims. drawing Fig. 1 is an elevational view partially in section of a cathode ray tube embodying my invention and Figs. 2 to 4 inclusive are enlarged views in section illustrating various steps in a process embodying my invention.

Referring now to the drawing, there is illustrated in Fig. l a cathode ray tube of a type to which my invention may be applied with particular advantage. The tube as illustated includes an envelope having an enlarged bulbous portion I and an elongated shaft portion 2. The latter portion contains an electron gun 3 for generating an electron beam adapted to be projected longitudinal of the envelope. A conducting coating 4 which may, for example, be an aqueous suspension of graphite, is applied to the inner surface of the envelope. As illustrated in the drawing, the coating may extend from the large end of the envelope substantially to the electron gun and provides an accelerating electrode by means of which an appropriate axial electric field may be produced within the envelope. The large end of the envelope provides a transparent window 5 which is provided on its inner surface with a layer 6 of a fluorescent ma- In the I terial which is adapted to be excited to visible luminescence by the impingement of electrons. Suitable electron deecting means illustrated diagrammatically as electromagnetic deflecting coils l and 8 are provided for the purpose of ycausing the electron beam to scan .elemental areas of the screen .to develope a picture thereon.

In the operation of tubes of the type just `described, a dark spot tends to form on the center [portion of the fluorescent screen. The darkening in the screen has 'been attributed to the presence of ions in the electron beam which are not deected by the electromagnetic delecting means and which tend to burn the uorescent material. These ions may be prevented from producing this detrimental lefiect by providing a metal nlm on the back of the fluorescent material. This film if imperforate and thin permits the electrons to pass through to the screen substantially unimpeded -w-hi'le .collecting substan.- tially all of the ions. A conducting backing also eliminates a tendency for charges to accumulate cn the screen and -to minimize :the projection of light from the screen yto the interior of the tube envelope.

In accordance with my invention, the metal backing which maybe -of such materials as aluminum or beryllium is applied to the granular fluorescent coating by the successive steps illustrated in Figs. 2 to 4 inclusive. In Fig. 2 an enlarged section of the Window of the tube is illustrated at 5 and the granular structure of the fluorescent screen is designated by the numeral 6'. In the rst step of the process a controlled amount of liquid is applied to the fluorescent screen so that the interstices are just lled with the liquid as illustrated at 9. While a number of liquids may be employed, I have found that water is particularly suitable. The amount of water may be controlled by applying a measured quantity to a tube of given dimensions or as I have found preferable an excess of Water may be introduced into the interior of the tube and flushed over the fluorescent screen. The tube envelope may then be inverted and the excess water drained oii. The removal of the excess may be accelerated by the application of heat if desired. Some experimentation is required with tubes of different sizes and with screens having different granular screen structures to determine the exact technique required to provide just enough liquid on the screen to fill the interstices.

The next step of the process is accomplished by flowing over the surface provided by the screen and water, a very thin layer of a filmforming resin dissolved in a volatile solvent. A large number of synthetic resins are suitable for this purpose, and I have found that a resin produced by the reaction of an aldehyde with a hydrolyzed polymerized vinyl ester is particularly suitable for this process. The resin which is insoluble in Water is dissolved in a suitable solvent which may, for example, be ethylene dichloride. The solution is flushed over the surface to provide a thin film, for example, 500 to 10,000 angstrom units, and the film allowed to set (by evaporation of the solvent). The surface of the resin layer I thus applied is very smooth and provides a suitable carrier on which the metal backing may be deposited. The metal may be deposited on the resin iilm by a suitable evaporation process to provide a thin layer of suitable thickness as designated in Fig. 4 by numeral ll. The metal film produced is of uniform thickness and relatively free from small holes. In the completion of the tube the screen is heated to a relatively high temperature, for example, in the order of 400 C. This heating removes any residual moisture and also removes substantially all the resin film, leaving the thin metallic coating over the back of the screen. This heating may be performed in air or vacuum. Baking out in vacuum yields films of excellent reiiectivity. Air baking decreases the reiiectivity somewhat, probably about 20 per cent reduction.

The method described above in connection with the manufacture of cathode ray tubes for television purposes may be applied to the coating of other granular materials with a thin impervious metal coating. When used for television tubes the metal iilms improve contrast very considerably as light output is increased and no back lighting results.

While I have described and illustrated a particular embodiment of my invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects and I therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention..

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of producing a smooth thin metal backing on a granular fluorescent screen which comprises the steps of applying sufiicient Water to the screen to fill the intergranular spaces, covering the screen and water with a layer of a solution of water-insoluble synthetic heat removable resin in a volatile non-aqueous solvent to a thickness in the order of 500 to 10,000 angstrom units, evaporating the solvent to cause the resin to set, depositing a thin layer of metal on the resin by vaporizing the metal and baking the screen at a temperature in the order of 400 C. to remove the resin and complete the metal backed fluorescent screen.

2. The method of producing a smooth thin metal backing on a granular fluorescent screen which comprises the steps of applying suflicient water to the screen to fill the intergranular surface, covering the screen and water with a layer of a solution of a water-insoluble synthetic heat removable resin in a volatile non-aqueous solvent to a thickness in the order of 500 to 10,000 angstrom units, said resin being the reaction product of an aldehyde with a hydrolyzed polymerized vinyl ester, allowing the resin to set by vaporization of the solvent, depositing a thin layer of metal on the resin by vaporizing the metal on the resin layer and baking the screen to a temperature in the order of 400 C. to remove the resin and complete the metal backed fluorescent screen.

VON C. CAMPBELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,233,786 Law Mar. 4, 1941 2,285,424 Fenner June 9, 1942 2,373,849 Palmer Apr. 17, 1945 2,374,310 Schaeffer Apr. 24, 1945 2,400,304 Hamel May 14, 1946 

1. THE METHOD OF PRODUCING A SMOOTH THIN METAL BACKING ON A GRANULAR FLUORESCENT SCREEN WHICH COMPRISES THE STEPS OF APPLYING SUFFICIENT WATER TO THE SCREEN TO FILL THE INTERGRANULAR SPACES, COVERING THE SCREEN AND WATER WITH A LAYER OF A SOLUTION OF WATER-INSOLUBLE SYNTHETIC HEAT REMOVABLE RESIN IN A VOLATILE NON-AQUEOUS SOLVENT TO A THICKNESS IN THE ORDER OF 500 TO 10,000 ANGSTROM UNITS, EVAPORATING THE SOLVENT TO CAUSE THE RESIN TO SET, DEPOSITING A THIN LAYER OF METAL ON THE RESIN BY VAPORIZING THE METAL AND BAKING THE SCREEN AT A TEMPERATURE IN THE ORDER OF 400* C. TO REMOVE THE RESIN AND COMPLETE THE METAL BACKED FLUORESCENT SCREEN. 